In order to maintain networks or communication systems properly, for example, telephone transmission facilities and the like, numerous measurements are made of network and system characteristics. Important among these are the measurements of frequency response and envelope delay distortion. To this end, what is commonly called envelope delay is measured over the frequency range of the facility being evaluated. Envelope delay is defined as the slope of the phase versus frequency characteristic of the transmission facility. In an ideal communications system, envelope delay is constant over the frequency band. However, in practical systems there are deviations in the envelope delay over the frequency band. These deviations from an arbitrary reference are defined as the envelope delay distortion of the facility.
Heretofore, envelope delay measurements have been made by employing a carrier frequency signal which is amplitude modulated by a stable "low" frequency reference signal. The carrier frequency and upper and lower sidebands are propagated through the facility being evaluated, thereby experiencing a delay dependent upon their position in the frequency band. These signals are detected at the output of the facility under evaluation. Then, a measure of envelope delay at the carrier frequency is obtained by precisely measuring the delay interval between the detected signals and the low frequency reference signal. Thereafter, the carrier frequency is stepped or swept across the frequency band to obtain an overall measure of envelope delay distortion over the frequency band of interest. One such measurement system is disclosed in U.S. Pat. No. 3,271,666 issued to T. C. Anderson and D. L. Favin on Sept. 6, 1966.
More recently, envelope delay measurements have been attempted employing a test signal including a plurality of tones or tone pairs spaced at predetermined frequencies in the frequency band of interest in order to simultaneously obtain a measure of envelope delay over the entire frequency band for the facility under evaluation, as described in U.S. Pat. No. 3,573,611, issued Apr. 6, 1971.
Problems common to the known prior measurement arrangements are errors in and repetition of measurements of envelope delay distortion of facilities on which noise, frequency shift, nonlinearity or other impairments are present. Moreover, it is important to obtain accurate and reliable measurements in the presence of subtle changes in the amount of intermodulation distortion on the facility under evaluation.
Data collected for obtaining envelope delay measurements is also utilized to compute the frequency response of the facility.